Sennheiser MKH 20 Service Manual page 8

The amplifier output signal is supplied to the microphone output via C17, C18, R20,
R21, and the RF barrier L6, L7, and C19 to C22. The output impedance is mainly
determined by R20, R21, and the resistive portions of L6 and L7, and is about 150Ω
in the center frequency range. The Z diodes D4 to D6 protect C16 to C18 from
overvoltages.
Output circuit
During phantom supply, each of the two cable strands supplies half of the
operating current to the microphone. The divided operating current is merged by
the two resistors R20 and R21 (or R18, R19) with identical resistances. The current
flows back to the phantom supply source via the amplifier circuit (MKH60...70: via
both amplifier stages), the oscillator, and the cable shield (microphone ground).
Since the operating current is stabilized by the oscillator circuit, the AC side of the
amplifier circuit is decoupled from ground ("electronic choke") and the available
output signal is floating. The signal is balanced by means of the two supply
resistors (6.8kΩ each) of the phantom supply source.
MKH 20...70
8/53 04/2005